A series circuit, antenna-antenna switch-Rx filter-matching circuit-LNA-mixer, is usually present in the receive path, referred to as the Rx path, of cell phones. For the front end of cell phones, which contains at least part of the Rx path, complete chipsets or even modules are offered that comprise individual components matched to one another, or are integrated circuits, or contain several or all components integrated on one module. Because of their reasonable prices, CMOS circuits are being increasingly used for the LNA (Low Noise Amplifier). Such LNAs have input impedances of, for instance, 800Ω or more. They have at least one bad noise figure, so that an input signal with as high an amplitude as possible, i.e., low attenuation, is required to meet the requirements of the overall system.
SAW filters with a real output impedance of typically up to 200Ω are used for the Rx branch and also for the TX branch for cell phones. Matching to the LNA with the higher input impedance is achieved with external components, which, however, have finite figures of merit and therefore produce losses. Integrating these matching components entirely or partially into modules is a known measure for minimizing these losses. Nevertheless, it is necessary to accept losses of typically 2.0 dB in the front end of cell phones from the SAW filter alone, and typically ca. 1.0 dB in addition from the external matching for impedance transformation to the actual 800Ω.
A DMS filter in which the impedance of the input or output filter is determined by weighting is known from U.S. Pat. No. 6,353,372 A. This transducer is divided into two partial transducers connected in series at least in part. The division of the input or output transducer into two partial transducers can be done horizontally, the partial transducers then being arranged side by side transversally in the acoustic path. It is also possible to divide the partial transducers vertically into two partial transducers, the partial transducers then being arranged side by side longitudinally inside the acoustic path. In case of a division into two partial transducers in which the size of the overall transducer, including the number of fingers, remains unchanged, one then achieves an impedance transformation between input side and output side by a factor of 4 in both cases.
To match an input filter between an antenna with a 50Ω impedance and an LNA with an input impedance of 800Ω, however, a higher impedance transformation, which cannot be achieved with known SAW filters, is necessary.